Authentication in on-premises and hybrid environments is challenging due to layered constraints such as firewalls and NATs, air-gapped networks, custom configurations, legacy protocols, and fragmented infrastructure. Common pitfalls include assuming public endpoints are always available, hardcoding auth protocol assumptions, neglecting certificate and key rotation, forgetting about auditing and monitoring, and overcomplicating the deployment model. Best practices for on-prem and hybrid authentication involve designing for asynchronous and fault-tolerant workflows, prioritizing end-to-end security, offering deployment flexibility, investing in documentation and self-service tools, planning for lifecycle management, and embracing operational excellence and customer empathy. WorkOS can help by providing a unified API for every identity provider, secure network integration, built-in certificate and metadata management, comprehensive observability and diagnostics, and deployment flexibility.